1. Field of the Invention
The present invention relates to the use of comb-branched copolymers as pigment dispersants, and more specifically to pigment dispersions containing pigment, a carrier medium, and a comb-branched copolymer pigment dispersant, and a method for making the pigment dispersions.
2. Background Art
Pigmented coating compositions are used in a wide variety of applications including paints, inks, and varnishes. The preparation of pigmented coating compositions generally involves mixing binder resin(s), crosslinker(s), additives, e.g., flow additives, and solvents with a compatible pigment dispersion. Pigment dispersions are typically prepared by mixing dry pigment (inorganic or organic) with a pigment dispersant in the presence of a carrier medium, e.g., an aqueous carrier such as water, or solvent based carriers.
Dry pigments are available commercially in the form of agglomerated pigment particles. Pigment agglomerates are more likely to settle out of pigment dispersions and/or pigmented coating compositions and are accordingly undesirable. To break the pigment agglomerates down into smaller agglomerates and/or individual particles generally requires a use of energy intensive mixing means (commonly referred to as grinding), e.g., sand mills and ball mills. During the grinding process, the pigment agglomerates are broken down into small agglomerates and/or individual particles the surfaces of which are wetted by the pigment dispersant. Pigment dispersants are provided in the pigment dispersion to suspend or disperse the pigment particles in the carrier medium and prevent their re-agglomeration on storage. It is desirable that the pigment dispersant function to effectively wet, disperse and stabilize both inorganic and organic pigments in either aqueous or non-aqueous solvents.
Some commercially available dispersants may act operatively for the above stated purposes, but disadvantageously create a relatively high amount of foam. In this context, foam is described as a frothy mass of fine bubbles formed in or on the surface of a liquid. A stabilized foam is usually the result of air entrainment in the liquid due to mechanical mixing. The problems of high levels of foam during the dispersion phase can be encountered in both the pre-mix and milling chambers of traditional dispersing equipment. The presence of foam slows down the dispersion process. Foam can also adversely affect film properties of derived coatings such as moisture resistance. In addition to being operable without generating high amounts of foam, suitable dispersants should be capable of forming stable pigment dispersions that have relatively high pigment loading capabilities.
Accordingly, it would be desirable to provide a pigment dispersant that forms the above needed functions of a pigment dispersant and foams less than other prior art dispersants.
It has now been surprisingly discovered that the above and other objectives can be met by providing a dispersant comprising an acrylic/polyether comb-branched copolymer.
It has been further surprisingly discovered that very stable dispersions having a dispersant comprising an acrylic/polyether comb-branched copolymer have relatively low foaming.
Accordingly, the present invention comprises a stable, low foaming dispersion comprising: a) pigment; b) carrier; and c) an acrylic/polyether comb-branched copolymer dispersant wherein the polyether portion of the copolymer is free of any acidic groups.
Moreover, the present invention comprises a method of making a dispersion suitable for use in pigmented coating compositions. The method includes mixing together, in any combination: a) pigment; b) carrier; and c) an acrylic/polyether comb-branched copolymer dispersant wherein the polyether portion of the copolymer is free of any acidic groups.
Furthermore, the present invention comprises a pigment coating composition comprising the dispersion described above.
Dispersions made in accordance with the present invention comprise, at a minimum pigment, carrier, and a dispersant comprising an acrylic/polyether comb-branched copolymer.
The acrylic/polyether comb-branched copolymer preferably has a molecular weight of 400 grams per mole to about 1,000,000 grams per mole, more preferably between about 600 grams per mole to about 800,000 grams per mole, and most preferably between about 1,000 grams per mole to about 600,000 grams per mole. The copolymer preferably has a mole ratio of acrylic monomer units to polyether units of about 1/99 to about 99/1, more preferably from about 1/1 to about 20/1, and most preferably from about 2/1 to about 20/1. The pendant polyether chain of the copolymer is free of any acidic groups, preferably free of any ionic groups, and more preferably free of any phosphate groups.
The comb-branched copolymer can be made by any suitable process for copolymerizing acrylic units with polyether units. In one preferred method, the copolymer is formed by reacting a polyether polymer or macromonomer with a polyacrylic acid polymer or acrylic monomer. The process may be continuous, batch, or semi-batch. Following the copolymerization process, any relatively volatile unreacted monomers are generally stripped from the product.
More preferably, the comb-branched copolymer is made according to one of the three following processes. The first process (i) comprises copolymerizing an unsaturated polyether macromonomer with at least one ethylenically unsaturated comonomer selected from the group consisting of carboxylic acids, carboxylic acid salts, hydroxyalkyl esters of carboxylic acids, and carboxylic acid anhydrides. The second process (ii) comprises reacting a carboxylic acid polymer and (a) a polyether prepared by polymerizing a C2-C4 epoxide or (b) a polyether mixture comprising (1) a monofunctional polyether prepared by polymerizing a first epoxide selected from the group consisting of C2-C4 epoxides and mixtures thereof onto a monofunctional initiator and (2) a difunctional polyether prepared by polymerizing a second epoxide selected from the group consisting of C2-C4 epoxides and mixtures thereof, which may be the same as or different from the first epoxide, onto a difunctional initiator wherein the carboxylic acid polymer and the polyethers are reacted under conditions effective to achieve partial cleavage of the polyether and esterification of the polyether and cleavage products thereof by the carboxylic acid polymer. The third process (iii) comprises polymerizing a polymerizable acid monomer containing at least one ethylenically unsaturated group in conjugation with a carboxyl group selected from the group consisting of carboxylic acid, carboxylic anhydride and carboxylic ester groups in a reaction medium comprising a polyether, wherein the polyether is prepared by polymerizing a C2-C4 epoxide, to form a carboxylic acid polymer; and reacting the carboxylic acid polymer and the polyether under conditions effective to achieve esterification of the polyether by the carboxylic acid polymer to form the comb-branched copolymer.
The preferred polyether macromonomer preferably comprises ethylene oxide and propylene oxide and has a molecular weight of about 300 grams per mole to about 100,000 grams per mole, more preferably between about 500 grams per mole to about 75,000 grams per mole, and most preferably between about 1,000 grams per mole to about 10,000 grams per mole. All molecular weights are number average molecular weights unless stated otherwise. Preferably, the ratio of propylene oxide (PO) to ethylene oxide (EO) of the polyether polymer or polyether macromonomer is between about 99/1 to about 1/99, more preferably between about 95/5 to about 1/99, and even more preferably between about 80/20 to about 1/99 by weight, and most preferably between about 50/50 to about 30/70.
A preferred process for making the copolymer comprises: (a) forming a monomer stream, an initiator stream, and an optional chain transfer agent stream; (b) polymerizing the streams in a reaction zone at a temperature within the range of about xe2x88x9220xc2x0 C. to about 150xc2x0 C.; and (c) withdrawing a polymer stream from the reaction zone. This process is described in more detail in copending U.S. patent application Ser. No. 09/358,009, filed Jul. 21, 1999, now U.S. Pat. No. 6,214,958, which is incorporated herein by reference.
The monomer stream contains an acrylic monomer and a polyether macromonomer. Suitable acrylic monomers are derived from acrylic acid and methacrylic acid. Preferred acrylic monomers include acrylic acid, methacrylic acid, their ammonium and alkali metal salts, their C1 to C10 alkyl and C6 to C12 aryl esters, and their amides. Acrylic acid, methacrylic acid, ammonium acrylate, ammonium methacrylate, sodium acrylate, sodium methacrylate, potassium acrylate, and potassium methacrylate are preferred. Most preferred are acrylic acid and methacrylic acid.
Suitable polyether macromonomers have a polyether chain and a single carbon-carbon double bond, which can be located either terminally or within the polyether chain. Examples include polyether monoacrylates, polyether monomethacrylates, polyether monoallyl ethers, polyether monomaleates, and polyether monofumarates. Further examples include the reaction product of a hydroxyl-functional polyether with isocyanatoalkyl(meth)acrylates such as isocyanatoethylacrylate, and with ethylenically unsaturated aryl isocyanates. The polyether of the macromonomer is an alkylene oxide polymer having a number average molecular weight within the range of about 500 to about 10,000. Suitable alkylene oxides include ethylene oxide, propylene oxide, butylene oxide, and the like, and mixtures thereof. The polyether macromonomers preferably have hydroxyl functionality from 0 to 5. They can be either linear or branched polymers, homopolymers or copolymers, random or block copolymers, diblock or multiple-block copolymers.
Examples of polyether macromonomers are poly(propylene glycol) acrylates or methacrylates, poly(ethylene glycol) acrylates or methacrylates, poly(ethylene glycol) methyl ether acrylates or methacrylates, acrylates or methacrylates of an oxyethylene and oxypropylene block or random copolymer, poly(propylene glycol) allyl ether, poly(ethylene glycol) allyl ether, poly(propylene glycol) monomaleate, and the like, and mixtures thereof. Preferred polyether macromonomers are poly(propylene glycol) acrylates or methacrylates, poly(ethylene glycol) acrylates or methacrylates, acrylates or methacrylates of an oxyethylene and oxypropylene block and/or random copolymer. More preferred are acrylates or methacrylates of an oxyethylene and oxypropylene block and/or random copolymer.
The ratio of acrylic monomer to polyether macromonomer is determined by many factors within the skilled person""s discretion, including the required physical properties of the comb-branched copolymer, the selection of the acrylic monomer, and the properties of the polyether macromonomer. The ratio generally is within the range from 1/99 to 99/1 by weight. The preferred range is from 5/95 to 75/25.
In one embodiment, the macromonomer is made by (a) oxyalkylating an initiator molecule selected from the group consisting of hydroxyalkyl acrylates, hydroxyalkyl methacrylates, and monounsaturated monocarboxylic acids with an alkylene oxide in the presence of an effective amount of a double metal cyanide complex catalyst under conditions effective to form a well-defined unsaturated macromonomer having a terminal hydroxyl functionality and not more than substantially one initiator molecule per unsaturated macromonomer molecule. This method is described in substantial detail in U.S. Pat. No. 6,034,208, which is incorporated herein by reference. Also, the macromonomer described in U.S. Pat. No. 6,034,208 in addition to being reacted in the manner described in the preferred continuous process described herein, can be reacted with the comonomer in the manner described in U.S. Pat. No. 6,034,208.
Optionally, the monomer stream contains a third monomer. The third monomer is preferably selected from vinyl aromatics, vinyl halides, vinyl ethers, vinyl esters, vinyl pyrrolidinones, conjugated dienes, unsaturated sulfonic acids, unsaturated phosphonic acids, and the like, and mixtures thereof. The amount of third monomer used depends on the required physical properties of the comb-branched copolymer product, but is preferably less than 50% by weight of the total amount of monomers.
Optionally, the monomer stream also includes a solvent. The solvent is used to dissolve the monomer, to assist heat transfer of the polymerization, or to reduce the viscosity of the final product. The solvent is preferably selected from water, alcohols, ethers, esters, ketones, aliphatic hydrocarbons, aromatic hydrocarbons, halides, and the like, and mixtures thereof. Selections of solvent type and amount are determined by the polymerization conditions including reaction temperature. Water and alcohols, such as methanol, ethanol, and isopropanol are preferred.
The initiator stream contains a free radial initiator. The initiator is preferably selected from persulfates, hydrogen peroxide, organic peroxides and hydroperoxides, azo compounds, and redox initiators such as hydrogen peroxide plus ferrous ion. Persulfates, such as ammonium and potassium persulfate, are preferred.
Optionally, the initiator stream contains a solvent. The solvent is used to dissolve or dilute the initiator, to control the polymerization rate, or to aid heat or mass transfer of the polymerization. Selections of solvent type and amount are determined by the nature of the initiator and the polymerization conditions. Water and alcohols such as methanol, ethanol, and isopropanol are preferred when persulfate is used as the initiator.
The monomer and initiator streams optionally include a chain transfer agent. Suitable chain transfer agents include alkyliodides and bromides, branched lower alcohols such as isopropanol, alkyl amines, alkyl sulfides, alkyl disulfides, carbon tetrahalides, allyl ethers, and mercaptans. Mercaptans, such as dodecyl mercaptan, butyl mercaptan, mercaptoacetic and mercaptopropionic acids, are preferred.
Under some conditions, it is preferred to add the chain transfer agent in a separate stream. This is particularly desirable when the chain transfer agent causes decomposition of the initiator or polymerization of the monomer once it is mixed with those components. This is particularly important in a large, commercial scale because these reactions can cause safety problems.
Optionally, the chain transfer agent stream contains a solvent that is used to dissolve or dilute the chain transfer agent. Suitable solvents include water, alcohols, ethers, esters, ketones, aliphatic and aromatic hydrocarbons, halides, and the like, and mixtures thereof. Selections of solvent type and amount are determined by the nature of the chain transfer agent and the polymerization conditions. Water and alcohols, such as methanol, ethanol, and isopropanol, are preferred.
The monomer stream, initiator stream, and optional chain transfer agent stream are polymerized in a reaction zone. The reaction temperature is preferably kept essentially constant during the polymerization. The temperature is determined by a combination of factors including the desired molecular weight of the comb-branched polymer product, the initiator type and concentration, the monomer type and concentration, and the solvent used. The reaction is performed at a temperature within the range of about xe2x88x9220xc2x0 C. to about 150xc2x0 C., preferably, within the range of about 20xc2x0 C. to about 90xc2x0 C. Most preferred is the range of about 40xc2x0 C. to about 60xc2x0 C.
The addition rate of each stream depends on the desired concentration of each component, the size and shape of the reaction zone, the reaction temperature, and many other considerations. In general, the streams flow into the reaction zone at rates that keep the initiator concentration within the range of about 0.01% to about 1% by weight, and the chain transfer agent concentration within the range of about 0.1% to about 1.5% by weight.
The reaction zone is where the polymerization takes place. It can be in the form of a tank reactor, a tubular reactor, or any other desirably shaped reactor. The reaction zone is preferably equipped with a mixer, a heat transfer device, an inert gas source, and any other suitable equipment.
As the streams are polymerized in the reaction zone, a polymer stream is withdrawn. The flow rate of the polymer stream is such that the reaction zone is mass-balanced, meaning that the amount of material that flows into the reaction zone is equal to the amount of material withdrawn from the reaction zone. The polymer stream is then collected.
The comb-branched copolymer may also be made according to a multiple-zone process. A multiple-zone process is similar to the process discussed above except that more than one reaction zone is used. In a multiple-zone process, a first polymer stream is withdrawn from a first reaction zone and transferred into a second reaction zone where the polymerization continues. A second polymer stream is withdrawn from the second reaction zone. More than two reaction zones can be used if desirable. The reaction temperature in the second reaction zone can be the same as or different from the first reaction zone. A multiple-zone process can enhance monomer conversion and increase efficiency of the process. Usually, in the first polymer stream, the monomer conversion is within the range of about 65% to 85% by weight. The second reaction zone preferably brings the monomer conversion to 90% or greater.
In a second preferred process, the comb-branched copolymer used in accordance with the present invention can be made by reacting (a) a carboxylic acid polymer and (b) a polyether macromonomer prepared by polymerizing a C2-C4 epoxide or (c) a polyether mixture comprising (1) a monofunctional polyether prepared by polymerizing a first epoxide selected from the group consisting of C2-C4 epoxides and mixtures thereof onto a monofunctional initiator and (2) a difunctional polyether prepared by polymerizing a second epoxide selected from the group consisting of C2-C4 epoxides and mixtures thereof, which may be the same as or different from the first epoxide, onto a difunctional initiator wherein the carboxylic acid polymer and the polyethers are reacted under conditions effective to achieve partial cleavage of the polyether and esterification of the polyether and cleavage products thereof by the carboxylic acid polymer. These methods are described in substantial detail in U.S. Pat. Nos. 5,614,017 and 5,670,578 which are incorporated herein by reference.
In a third preferred process, the comb-branched copolymer used in accordance with the present invention can be made by polymerizing a polymerizable acid monomer containing at least one ethylenically unsaturated group in conjugation with a carboxyl group selected from the group consisting of carboxylic acid, carboxylic anhydride and carboxylic ester groups in a reaction medium comprising a polyether, wherein the polyether is prepared by polymerizing a C2-C4 epoxide, to form a carboxylic acid polymer; and reacting the carboxylic acid polymer and the polyether under conditions effective to achieve esterification of the polyether by the carboxylic acid polymer to form the comb-branched copolymer. This method is described in substantial detail in U.S. Pat. No. 5,985,989 which is incorporated herein by reference.
The dispersant is typically present in the pigment dispersion of the present invention in an amount of at least 0.02 percent by weight, preferably at least 0.05 percent by weight, and more preferably at least 0.1 percent by weight, based on the total weight of the pigment. The dispersant is also typically present in the dispersion in an amount of less than 65 percent by weight, preferably less than 40 percent by weight, and more preferably less than 30 percent by weight, based on the total weight of the pigment. The amount of dispersant present in the dispersion of the present invention may range between any combination of these values, inclusive of the recited values.
The pigment of the dispersion of the present invention may be selected from inorganic pigments (such as carbon black pigments, e.g., furnace blacks, electrically conductive carbon black pigments, extender pigments and corrosion inhibitive pigments); organic pigments; dispersed dyes; and mixtures thereof. Examples of organic pigments that may be present in the pigment dispersion include, but are not limited to, perylenes, phthalo green, phthalo blue, nitroso pigments, manoazo pigments, diazo pigments, diazo condensation pigments, basic dye pigments, alkali blue pigments, blue lake pigments, phloxin pigments, quinacridone pigments, lake pigments of acid yellow 1 and 3, carbozole dioxazine violet pigments, alizarine lake pigments, vat pigments, phthaloxy amine pigments, carmine lake pigments, tetrachloroisoindolinone pigments and mixtures thereof. Inorganic pigments that may be present in the pigment dispersion, include, for example, titanium dioxide, electrically conductive titanium dioxide, and iron oxides, e.g., red iron oxides, yellow iron oxides, black iron oxides and transparent iron oxides. Extender pigments that may be present in the pigment dispersion include, but are not limited to, silicas, clays, alkaline earth metal sulfates and carbonates, such as calcium sulfate, magnesium sulfate, barium sulfate, and calcium carbonate. The pigment dispersion may contain corrosion inhibitive pigments, such as aluminum phosphate and calcium modified silica.
The pigment is typically present in the dispersion of the present invention in an amount of at least 0.5 percent by weight, more typically at least 5 by weight, and even more typically at least 20 percent by weight, based on the total weight of the dispersion. While the pigment loading could preferably be even higher, the pigment is also typically present in the dispersion in an amount of less than 95 percent by weight, more typically less than 90 percent by weight, and even more typically less than 75 percent by weight, based on the total weight of the dispersion. The amount of pigment present in the dispersion may range between any combination of these values inclusive of the recited values.
The pigment and dispersant are typically present together in the dispersion in an amount totaling from 0.5 percent by weight to 95 percent by weight, more typically from 5 percent by weight to 95 percent by weight, and most typically from 10 percent by weight to 95 percent by weight, based on the total weight of the dispersion. The percent weights are based on the total combined weight of the pigment and dispersant. The weight ratio of pigment to dispersant is typically from 1.5/1 to 5,000/1, more typically from 2.5/1 to 2,000/1, or most typically from 3.3/1 to 1,000/1.
The dispersion of the present invention also comprises a carrier selected from water, organic solvent, and a mixture of water and at least one organic solvent (preferably a water soluble organic solvent). Suitable classes of organic solvents that may be used include, but are not limited to, alcohols, e.g., methanol, ethanol, n-propanol, iso-propanol, n-butanol, sec-butyl alcohol, tert-butyl alcohol, iso-butyl alcohol, furfuryl alcohol and tetrahydrofurfuryl alcohol; ketones or ketoalcohols, e.g., acetone, methyl ethyl ketone, and diacetone alcohol; ethers, e.g., dimethyl ether and methyl ethyl ether; cyclic ethers, e.g., tetrahydrofuran and dioxane; esters, e.g., ethyl acetate, ethyl lactate, ethylene carbonate and propylene carbonate; polyhydric alcohols, e.g., ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, tetraethylene glycol, polyethylene glycol, glycerol, 2-methyl-2,4-pentanediol and 1,2,6-hexanetriol; hydroxy functional ethers of alkylene glycols, e.g., butyl 2-hydroxethyl ether, hexyl 2-hydroxyethyl ether, methyl 2-hydroxypropyl ether and phenyl 2-hydroxypropyl ether; nitrogen containing cyclic compounds, e.g., pyrrolidone, N-methyl-2-pyrrolidone and 1,3-dimethyl-2-imidazolidinone; and sulfur containing compounds such as thioglycol, dimethyl sulfoxide and tetramethylene sulfone.
When the carrier comprises a mixture of water and organic solvent, the aqueous carrier typically contains from 30 to 95 percent by weight of water, and from 5 to 70 percent by weight of organic solvent, e.g., from 60 to 95percent by weight of water, and from 5 to 40percent by weight of organic solvent. The percent weights are based on the total weight of the aqueous carrier.
The carrier is typically present in the dispersion of the present invention, in an amount of at least 5 percent by weight, more typically at least 10 percent by weight, and even more typically at least 15 percent by weight, based on the total weight of the dispersion. The carrier is also typically present in the dispersion in an amount of less than 99.5 percent by weight, and more typically less than 95 percent by weight, based on the total weight of the dispersion.
The dispersants of the present invention are especially suitable for preparing pigment dispersions by conventional dispersion techniques well known in the art such as roller mills, ball mills, Cowles dissolver, sand mills and others. The pigment is added to the dispersant, in the presence of a suitable liquid carrier, which may be a solvent, a reactive diluent or even another polymer so that the pigment dispersion has an appropriate viscosity for grinding and dispersing the pigment and maintaining it in a stable dispersed state. The polymeric dispersants of this invention allow pigments to be more readily dispersed in pigment grinds, without the need to use any other surfactant.
The pigment dispersions can contain other additives commonly used in pigment dispersions, for example, plasticizers, wetting agents, defoamers, diluents, and flow control agents.
The dispersants of the present invention contribute to pigment dispersions that are stable, have good color properties and have relatively high pigment loading capabilities.
The dispersion of the present invention is useful in the preparation of, for example, coating compositions, such as, paints, inks, and varnishes.